Besides sound absorbers in which energy is withdrawn from sound waves by gas friction in porous material, and transformed into heat, sound absorbers are known which dynamically absorb sound. In engineering, they are designated as acoustic absorbers; in physics, they are designated as dynamic absorbers. These are resonant systems which very well absorb sound occurring at their eigenfrequency and afterwards dissipate its energy. Examples for these resonant systems are so-called hole or Helmholtz resonators.
It also belongs to the known measures in the field of dynamic sound absorbers to build up acoustic resonators with the aid of one or more cross-sectional steps, i.e. by variations of the cross-section of a tube delimiting a gas channel. By means of these measures, however, the flow resistance for the gas flowing through the sound absorber is dramatically increased.
From DE 196 44 089 A1 a sound absorber for combustion engines is known which comprises a helical fixture provided in a gas channel through which a flowing gas passes, the helical fixture defines a helical gas passage through the gas channel. The sound shall be reduced by reflections and scattering at the helical surfaces of the helical fixture as well as by following absorption in the channel wall of the gas channel. Further, propagation of the sound shall be hindered by the so-called cut-off effect. For absorption of the sound in the channel wall the gas channel is enclosed by a ring channel which communicates with the gas channel via perforations, and in which a sound absorbing material, like for example ceramic wool, is arranged. Tuning to one or more main sound frequencies, i.e. a particularly high efficiency at these main sound frequencies, is not possible with the known sound absorber.
From DE 199 32 714 A1 it is known to arrange a helical membrane which is provided with anti-sound producing elements in a tube-shaped device through which air flows to actively dampen sound in the tube-shaped device. Active sound dampening with anti-sound producing elements, however, requires an active control of these elements and is correspondingly complex.
A device for reducing pressure pulsations in pipelines guiding liquids is known from DE 10 2004 006 031 A1. Here, a choke body is arranged in a pipeline of the pipeline system, which has a helix whose helix axis is oriented in a propagation direction of the pressure pulsations in the pipeline. The pressure pulsations interact with the helix. This may be a passive interaction under elastic deformation of the helix. Alternatively, the helix may be actively operated. One may also arrange a series of several choke bodies in the form of helixes at a fixed distance between the choke bodies in one pipeline. The teaching of DE 10 2004 006 031 A1, in contrary to the teaching of DE 199 32 714 A1, does expressively not relate to pipeline systems through which a flowing gas passes but only to pipeline systems guiding liquids.
An absorber for absorbing airborne sound in which an acoustic series circuit and an acoustic parallel circuit are coupled to each other, the acoustic series circuit being a Helmholtz resonator, is known from DE 195 33 623 B4. The Helmholtz resonator consists of a hollow body with an air volume and a reduction in cross-section as its opening. The parallel circuit also is a resonator which is realized by a parallel connection of an acoustic spring—realized by an air volume—and of an acoustic mass provided by the air oscillating in a neck. The acoustic series circuit and the acoustic parallel circuit are tuned to a same resonance frequency. No gas flows to the known absorber as such, but it is provided for making a generally gas tight wall in a sound absorbing way. The temporal entrance of gas into the hollow bodies or air volumes of the new absorber does—in contrary to a sound absorber through which gas flows—not result in a overall gas flow passing thereto.
GB 460,148 A discloses a sound absorber for combustion engines comprising several gas channels over which the flowing gas is distributed. In the interior of the gas channels helical fixtures can be provided. The helical fixtures may comprise intake and discharge areas at their end in which their diameter, in the flow direction of the gas, gets continuously closer from the inner to the wall of the gas channel, or gets farther away from it, respectively. The pitch of the helical fixtures of the known sound absorber may be variable. The gas channels may also comprise reductions or extensions of their free cross-section along their direction of main extension. The several gas channels are, for example, used for an extinguishing superposition of sound waves which propagate in the flowing gas.
There still is a need for a sound absorber which, by means of passive elements, comprises a high sound dampening as compared to the flow resistance for the flowing gas passing through the sound absorber.